How to draw a single graffiti over the multiple objects?
Use the 'Object' coordinates of the Texture Coordinates node with the 'Object' property set to the same object for each of your meshes. This will use the same coordinates for each mesh so that the texture can be mapped over all of them.
Here's an example :

Each of the cubes is a separate object - all with the same material. Note that the Texture Coordinate node is set to 'Cube' (one of the cubes) for the Object. This means that the Object coordinates are always with respect to that specific object. The Mapping simply allows you to scale and position the image.
The 'Separate XYZ' and 'Less Than' nodes use the Z-channel of the Generated coordinates so that the texture is only used for the upper surface of each cube (otherwise it would also show down the sides and on the bottom).